This invention concerns an apparatus for the measurement of flow rate of a flowing medium according to the ultrasonic Doppler method, which apparatus comprises an applicator with an ultrasonic generator and Doppler processing means for providing a Doppler flow measurement compensated for the ultrasonic radiation angle relative to the flow axis of the flowing medium.
In the case of flow measurements of flowing media, what is of primary interest is the velocity of flow or also the volume rate of flow, the latter, however, being readily calculated by means of product formation if the velocity of flow and the flow cross section are known. In all these measurements, however, there is the disadvantage that the frequency of the received Doppler signals which are used as the criterion for the velocity of flow, is not, as is actually desired, solely dependent upon the flow rate since this frequency is also dependent upon the irradiation angle of the ultrasonic energy relative to the flow axis of the flowing medium. In U.S. Letter Pat. No. 3,766,517, there is shown a prior art apparatus for the angle-independent measurement of flow rate, particularly that of the blood, wherein by means of the irradiation of ultrasonic energy from two transmission directions forming an angle of 90.degree., the effect of the angle between the irradiation axis and the flow axis is eliminated. Thus if the angles are designated .alpha. and 90 minus .alpha., the frequency shifts .DELTA. f.sub.1 and .DELTA. f.sub.2 due to the Doppler effect are equal to k cos .alpha., and k cos (90-.alpha.) or k sin .alpha.. If the signals .DELTA. f.sub.1 and .DELTA. f.sub.2 are squared and then summed, in suitable signal processing circuitry, the result may be expressed as: .DELTA. f.sub.1.sup.2 + .DELTA. f.sub.2.sup.2 = k.sup.2 (sin.sup.2 .alpha. + cos.sup.2 .alpha.). In this expression the radiation angle .alpha. is eliminated since sin.sup.2 .alpha. + cos.sup.2 .alpha. is equal to one. In addition, there is another prior art apparatus for the angle-independent measurement of flow rate, particularly of the blood, where two transmitter/receiver elements are arranged at a specific interval next to one another, and the time difference between the arrival of the ultrasonic energy at the first and second transmitter/receiver elements after impingement on the flowing medium is measured. (In this latter apparatus, the ultrasonic energy is radiated into the medium from the two sources simultaneously, and the time difference .DELTA. t is measured. The angle correction value k is calculated according to the equation ##EQU1## where c represents the velocity of the ultrasonic energy, and a represents the interval between the two transmitter/receiver elements.)
Both apparatus do indeed render possible an exact angle-independent measurement; however, they have the disadvantage that the angle-independence is achieved with relatively high electronic processing expenditures.